Keiji Ogura

Cyclic analysis of a propagating crack and its correlation with fatigue crack growth

Abstract Stress and strain field of a propagating fatigue crack and the resulting crack opening and closing behavior were analysed. It was found that a propagating fatigue crack was closed at tensile external loads due to the cyclically induced residual stresses. Strain range value Δϵ y in the vicinity of the crack tip was found to be closely related with the effective stress intensity factor range ΔK eff which was determined on the basts of the analytical crack opening and closing behavior at its tip. Application of this analysis to the non-propagating fatigue crack problem and the fatigue crack propagation problems under variable stress amplitude conditions revealed that both Δϵ y and ΔK eff were essential parameters governing fatigue crack growth rate.

Visualization of contact stress distribution using infrared stress-measurement system

A new experimental technique using an infrared stress measurement system and an infrared transmitting material is proposed for the visualization of a contact stress distribution. An infrared transmitting material is employed as one of two contacting materials, and is brought into contact with a cyclically loaded sample. Infrared emissions from the contact surface are measured through the infrared transmitting material, and thermoelastic stress analysis (TSA) is applied to measure the contact stress distribution. First, a flat contact is investigated, in which a barium- fluoride window is in contact with an embossed plastic letter. Stress distribution on the contact area in the letter can be measured accurately. Further, a spherical Hertz contact is examined by using an infrared transmitting sapphire convex lens and a flat plastic plate. The obtained contact stress distribution is compared with the stress distribution predicted by the Hertz theory. An excellent correspondence is found between those stress distributions, indicating the feasibility of quantitative analysis using the present technique.

Thermographic NDT based on transient temperature field under Joule effect heating

Thermographic NDT based on the transient temperature distribution under the Joule effect heating by an electric current was discussed. Two different types of inspection methods, the singular method and the insulation method, were examined. The singular method based on the heat concentration at the crack tip was successfully applied to the identification of the through- thickness and surface cracks embedded in steel plates. The resolution of the crack identification by the singular method was examined by the current intensity factor and the thermal diffusion length. The insulation method based on the temperature turbulence appears on the sample surface due to the thermal insulation of the defect was tested for the identification of the delaminated defect in CFRP. Two methods of the Joule effect heating, the direct current application and the induction heating, were successfully applied for the thermographic NDT of the delaminated defects in CFRP samples. The thermographic NDT developed in this study was found to be applicable to nondestructive flaw- and defect- inspection both in metallic and composite materials.

Application of Infrared Thermography for Contact Problems

Several experimental investigations of contact problems were made by the use of infrared thermography. A new experimental technique using infrared thermography combined with infrared transmitting materials was applied for continuous monitoring of the temperature distribution on the contact surface. Contact temperature measurement was examined for dry sliding contact of a plastic pin with an infrared transmitting disk. It was found that steady state contact surface temperature distribution was accurately monitored by thermography. Contact stress field was measured by an infrared stress measurement system based on the thermoelasticity through an infrared transmitting solid. It was found that the contact stress field was successfully visualized by the proposed method. Finally, a newly available infrared thermography system was used to characterize the near-surface conditions associated with fretting contact. It was found that both frictional temperature rise due to the interfacial global slip and the temperature fluctuation due to thermoelasticity can be measured to evaluate the nature of the contact stress field and the mechanics of partial slip fretting contacts.

Thermal sensing and imaging of the dry-sliding contact surface using IR thermomicroscope

A new experimental technique using an infrared (IR) thermography combined with IR transmitting materials is proposed for the visualization of the contact surface temperature of two solids. A dry sliding contact between a pin and a disk is investigated. IR transmitting solids such as sapphire, alumina ceramics and silicon are employed for the disk material. The temperature distribution on the contact surface is measured by the IR thermography through the IR transmitting disk. It is found that the temperature rise on the contact surface as well as the contact area is accurately analyzed from the obtained thermal images. The influence of contact pressure and sliding velocity on the contact surface temperature is discussed. Further, the dynamic process of wear of bearing steel against sapphire is investigated under dry sliding contact by the new type IRCCD thermography. Transient temperature distribution around the frictional heat spot was computed by the FEM analyses in order to estimate the magnitude of flush temperature from the experimental result in the dry sliding contact. All the results show that the proposed technique is useful for examining the successive process of the tribological phenomena.

On the three dimensional aspect of fatigue crack closure in plate specimens

In recent years much attention has been given to fatigue crack closure, but there are still unsolved problems on this phenomenon. One important problem to be examined would be such a three dimensional aspect on fatigue crack closure that the closure behavior on the surface of plate specimens may be much different from that in the interior. Lindrey and Richards [I] reported that closure is only a surface effect and no closure is found to exist beneath the surface. A similar trend of behavior is also observed by Pitoniak et al. [2].

Surface Strain Monitoring Approach to Delamination Failure of Thermal Barrier Coated Type 304 Stainless Steel under Thermal Cycle Condition.

Thermal cycle tests of thermal barrier coated type 304 stainless steel were conducted. 8 mass% Y2O3-ZrO2, Al2O3, CoNiCrAlY, NiCr coatings were used for the thermal barrier coatings (TBCs). Surface strain behavior during the thermal cycle test was measured using the laser speckle strain! displacement gauge (SSDG) in order to examine the delamination damage process. Surface strain was found to decrease when interface delamination occurred. Fatigue processes were discussed with the results of both surface strain measurement and microscopic observation of the specimen. Cracking occurred in ceramics layer and the crack grew parallel to the interface between the ceramics layer and the bond coating layer in the ceramics layer near the interface. The number of cycles to delamination decreased with increasing of the maximum temperature and hold time at its temperature. LPPS bond coating was found to be effective in extending the thermal cycle fatigue life of thermal barrier coated material.

Overview of recent Japanese activities in thermographic NDT

In the past decade, nondestructive testing techniques using infrared thermography, i.e., thermographic NDT techniques, received a lot of attention in many engineering fields in Japan. The first national symposium that specialized in thermographic NDT techniques was held in Tokyo, Japan on November 28-29, 1995, organized by the Research and Technical Committee on Surface Method of the Japanese Society for Nondestructive Inspection (JSNDI). At this symposium, twenty eight presentations including two keynote addresses were given. Over three hundred thermography researchers and engineers (thermographers) attended the symposium. Further, an exhibition of newly developed equipment for infrared thermography featuring the equipment of eleven companies took place concurrently. This symposium played an important role as the first national symposium dedicated to sharing information, ideas and experiences about thermographic NDT among thermographers from both the user and supplier sides. Sessions within the symposium were as follows: Advances in Infrared Imaging Systems; Applications for Composite Materials and Coated Materials; Diagnosis of Equipment/Monitoring, Applications for Structural Materials; Backup Techniques for Thermographic NDT; Infrared Stress Measurement and Contact Problems. This paper briefly describes presentations given in the symposium.

Damage inspection of CFRP using resistive-heating thermographic NDT

Thermographic NDT, which is a technique in NDT based on the surface temperature distribution in heated solids, was applied for the inspection of locally damaged CFRP plate samples under the lateral contact loading followed by cyclic bending. A singular method and an insulation method were examined. The singular method, in which a heat concentration at flaw tips was detected under resistive heating by electric current, was found to be sensitive to the failure, fracture or break in carbon fibers. On the other hand, the insulation method, in which the perturbation in the surface temperature distribution was detected under stream heating by hot air, was found to be successfully applicable to the inspection of the subsurface delamination damage in CFRP. The detected damages by the thermographic NDT were compared with those observed by SAM (scanning acoustic microscope).

Applications and Problems in an X-ray Fractographic Study of Fatigue Fracture Surfaces

ABSTRACT An X-ray fractographic study was made on fatigue fracture surfaces which were obtained from fatigue tests at room temperature, elevated temperature, and in a corrosive environment. Emphasis is put on examining the mechanical factors controlling the residual stress on the fatigue surfaces. The effect of fracture surface roughness and secondary damage on residual stress was also examined, and the problems and application limits of X-ray fractography to the fatigue fracture surface are discussed. The residual stress on the fracture surface was controlled by Kmax in the low Kmax (or ΔK) zone (Region I), while it was controlled by ΔK in the high Kmax (or ΔK) zone (Region III). For high-strength steel, another transitional region (Region II) was observed between these two regions. Increased fracture surface roughness beyond a certain value was detrimental to the fracture surface analysis, this value being estimated as about 7·5 μm of center-line average roughness. In this case, it was difficult to apply X-ray fractography because the residual stress was controlled not only by Kmax or ΔK but also by the roughness of the fracture surface. Even though a thin layer of oxide film or corrosion products is formed on the fracture surface at elevated temperatures or in a corrosive environment, X-ray fractography was as applicable to the fatigue fracture surface as one Produced under ambient conditions.